To date, weigh-in-motion (WIM) approaches attempt to weigh a vehicle while it is in motion by considering the vertical forces generated by the vehicle. In order for such approaches to be reliable, various attributes of the vehicle design need to be known ahead of time, such as the precise loading of each wheel, center of gravity, and/or the like. In practice, such knowledge cannot be accurately determined and utilized ahead of time, particularly in real-time applications. More importantly, adding a significant load to a vehicle can result in an often dramatic change to the attributes.
For current WIM systems to be usable, various restrictions on the installation and use of the system are applied. These restrictions include: requiring absolutely smooth and level pavement before and after the WIM system; requiring no turning, braking, or acceleration by the vehicle; limiting speeds to a specific target range; etc. Even with such restrictions, the accuracy of current WIM systems fail to meet reasonable requirements in many operating conditions. For example, the accuracy of: piezoelectric load (pressure) sensors is ±10%; bending plates is ±8%; and single load cells is ±6%. For a 60,000 pound vehicle, these errors can range from 3,600 pounds up to 6,000 pounds—equivalent to the weight of a large sport utility vehicle.
One approach seeks to account for oscillations that occur as a vehicle traverses a weighing system in order to provide a more accurate weight measurement. In this approach, oscillations in a single dimension are accounted for, but accurate measurement continues to require that the vehicles travel at low constant speeds with no turning or other factors.
Even across a relatively small subsection of vehicles, numerous parameters that can affect an accuracy of measuring the weight of the vehicle can vary substantially. For example, the table below illustrates the variation in several characteristics for vehicles weighing between roughly one and three tons.
Wheelbase2,347-4,000mmTrack width1,416-2,000mmCenter of Gravity (Z)540-1,000mmCenter of Gravity (X)1,063-1,478mmTire width185-315mmFront to back weight ratio53/47 to 66/34Front wheel weight range482-1784poundsRear wheel weight range433-1288poundsWhen considering all types of commercial vehicles, which can range in size from a panel truck to a double-length tractor trailer, the variability in these characteristics becomes immense.